Contaminated water, such as wastewater from hydrocarbon production operations or crude refining operations, needs to be treated prior to discharge into the environment, including when it is injected into an underground aquifer. Metal impurities in the wastewater generally need to be controlled to low levels. Many technologies are known to remove metal impurities from wastewater. Dense solid metal particles are often removed by allowing the wastewater to reside in tanks without stirring. The particles that are denser than water fall or settle to the bottom where they accumulate and can be removed. When metals are present in the ionic form, they can be removed by ion exchange or by precipitation and settling. Froth flotation and skimming is another known technology that can be used to remove the metal particles. Typically the process includes of a froth flotation vessel with a skimmer. This operation is typically referred to as “gas flotation” or “inert gas flotation.” The purpose of this operation is to remove oil from the water. The wastewater is sparged with an inert gas, and the oil accumulates on the surface where it is removed by a skimmer. Another operation to remove oil from water is a hydrocyclone.
In petroleum operations, i.e., crude oil refining or production, metal particles in wastewater that contain a relatively large amount of oil are not neutrally buoyant and will float to surface and can be removed by skimmers or hydrocyclones. Metal particles that contain a relatively low amount of oil are also not neutrally buoyant and will sink to the bottom and can be removed as sludge from the bottom of tanks. Metal particles containing or coated with oil can be neutrally, or near neutrally, buoyant. The oil is a crude oil or a crude oil component. Without the oil, the metal particles would be dense enough to settle, but in many cases, a proportion of the metal particles contain sufficient lower density oil that those aggregated oil-metal particles are neutrally buoyant. Ion exchange and settling are ineffective at removing neutrally buoyant metal particles. It is possible to remove such neutrally buoyant metal particles by filtration or centrifugation, but both of these processes are energy intensive and costly. Often the volumes of wastewater are large, making it impractical to use either filtration or centrifugation. A further difficulty in removing neutrally buoyant metal particles is the motion in the water due to the action of pumps and fluid flow through tanks and the like, preventing the particles from settling. What is needed is a process to remove neutrally buoyant metal particles from large volumes of water, e.g., wastewater, which is less energy intensive and costly than the currently known solutions.
In wastewater processing using oxidation ponds, a majority of the neutrally buoyant metal particles are removed. Without wishing to be bound by theory, it is believed that the microbes in the oxidation pond consume the oil in the aggregated oil-metal particles, rendering the particle no longer neutrally buoyant, so that the particle then sinks and becomes part of the biomass sludge. This approach removes a majority of the metal particles, but further removal can be required to meet discharge specifications. Additionally, the microbes in the oxidation pond are living organisms and can be compromised by variations in temperature, salinity, oxygen content, and toxins. When the microbes are compromised, the ability of the oxidation pond to remove the neutrally buoyant particles is also compromised.
What is needed is a process to augment or support the natural processes that occur in the oxidation ponds.